Craigslist doesn't support iFrame on their ads, which is necessary to embed an external site or landing page.
Using Landing Pages from Craigslist Ads can still be a good way to generate leads and/or sales. Since Craigslist doesn't allow you to embed the landing page, you can advertise your product or service and then include a hyperlink such as: Order Here, Learn More, etc.
You can do this by inserting this HTML where you want the hyperlink. For example:
Check out more product details here.<a href="http://yourlandingpage.com/">here</a>
What the ad visitor will see:
Check out more product details here.
You can change up the hyperlink text to whatever description you'd like.
Learn more about using HTML in Craigslist.